Data storage devices employ rotating data storage media such as hard disk drives. In a hard drive, data is written to the disk medium using a write head which generates a highly localized magnetic field which aligns magnetic domains within the disk in one of two directions, wherein one direction represents a “1” and the other direction represents a “0”. In some cases, the magnetization direction is up or down relative to the plane of the disk (perpendicular magnetic recording, or PMR). In other cases, the magnetization direction is within the plane of the disk. In all cases, this data may then be read-out with a read head. The write and read heads are typically integrated within a single assembly as shown in FIG. 21. To achieve steadily increasing data storage densities (typically measured in bits/inch2), which are now achieving levels near 1012 bits/in2, the sizes of magnetic regions storing individual bits have been reduced to nm levels. Writing to, and reading from, such small regions may include shrinking the sizes of the read and write heads and also having them “fly” closer to the disk surface (since the magnetic forces drop rapidly with increasing distance between the disk and the head). The distance between the head and the disk is called the “fly height” since the head is said to “fly” above the disk on a cushion of compressed air which is entrained by the rapid rotation of the disk and then squeezed between the head (often called a “sled”) and the disk. Very precise control of the fly height is achieved using “thermal fly height control” (TFC) which employs an electrical heater (with mW powers) to heat the pole pieces of the head, resulting in nm-level thermal expansion which pushes the pole pieces slightly closer to the spinning disk surface.
Precise control of the TFC power may include accurate and repeating time measurements of the fly height. For disk drives having multiple disks, and having multiple read/write heads (at least one for each disk surface), independent control of the TFC power to each head is typically employed since fly height variations may be uncorrelated between heads in the drive. Various methods for obtaining this fly height data at selected levels of precision and rapidity have been used, including the use of reference data tracks (with, for example, pure 140 MHz-written data), or using user data stored on the drive. An example of such a control method is described in U.S. patent application Ser. No. 13/211,593 which is incorporated in its entirety by reference herein. In all these methods, standard width data tracks were used, over which the read head may fly with an alignment (also called tracking) controlled by electronics within the disk drive. In particular, immediately following a seek operation in which the head is rapidly swept radially across a portion of the disk radius, the head may tend to oscillate from side-to-side (i.e., radially) relative to the data track for a period of time following the conclusion of the seek operation. This oscillation may result in the read head moving partially off the edges of the data track, with the result that the read signal (also called the readback signal, or read-out signal) is somewhat attenuated. In most cases, automatic gain control is capable of adjusting for these read signal fluctuations as far as data acquisition. A goal of some embodiments, however, is to employ this read signal to determine the fly height of the head, for which these read signal fluctuations may induce errors in the height measurement. This may occur because there are two motions which can cause read signal changes: side-to-side head motion (partially on and off the data track), and up-and-down motions corresponding to fly height variations. In a situation where both these effects may occur, separately, or together, it may be impossible to separate the effects of these two motions on the read signal intensity in order to obtain the desired fly height variation information.
A goal of some embodiments is to provide a method and structure for obtaining essentially unambiguous fly height information from the read-out signal, without interference from side-to-side head motions relative to the data track.
A further goal of some embodiments is to provide neighboring data tracks which overlap and are written with known reference data to provide a signal for use in controlling the fly height by means of thermal fly height control (TFC).
A still further goal of some embodiments is to use neighboring data tracks which overlap and are either unwritten, thermally erased, or AC demagnetized to provide a signal for use in controlling the fly height by means of thermal fly height control (TFC).